Human is a vehicle to Kubera, the lord of wealth and also the guardian of the north direction.
From Valmiki Ramayana, Aranya Kanda,
मत् भय आर्तः परित्यज्य स्वम् अधिष्ठानम् ऋद्धिमत् |
कैलासम् पर्वत श्रेष्ठम् अध्यास्ते नर वाहनः || ३-४८-५
I am that Ravana by whom Kubera, the god for Wealth-Management whose vehicular transport is handled by humans, is tormented fearfully and made to completely abandon his wealthiest place which is presided over by him till then, and I am the one who made that Kubera to flee to the excellent Mt. Kailash where he is somehow living, because I will not cause any harm on Mt. Kailash as I hold it sacred. [3-48-5]
The mounts of the Gods are explained in detailed in chapter 9 of Vamana Purana when a battle between Devatas and Daityas took place. While narrating the incident, Narada said to Pulastya:
रौद्रः शकतचक्राक्षः शैलाकारौ नरोत्तमम् |
अम्बिकापादसंभूतो वाहनम् धनदस्य तु ||
Narottama (meaning the best of human) having eyes like chariot wheels, a gigantic body as the mountain, a fierce figure was the carriage of Kubera, the lord of wealth. It originated from the feet of Ambika.(Here the first birth is told not at the time of churning of the ocean.)
Kubera is not only the one with Men as his carriage. The Yakshas(Demigods) also have men as Demigods.
कुञ्जरस्थाश्च वसवो यक्षाश्च नरवाहनाः|
किन्नरा भुजगारूढा हयारूढौ तथाश्विनौ ||
Elephants are the carriages of Vasus. Men are the carriages of demigods, serpents are the carriers of the eunuchs(Kinnaras) and horses were the carriages of Ashvini Devatas.
Why isn't a human the vahana of above mentioned Gods?
There will be a reason why Gods only chose them as the Vahanas. Having them as Vahanas(carriers) doesn't only mean that Gods are carried by that particular mount, but it also means that they have total control over it.
E.g : Shiva's vahana is an ox. To which the sanskrit word is Vṛṣabha.
Adi Shankaracharya while commenting on the name Vṛṣa in Vishnu sahasranama says:
The term Vrisha though not very familiar now is used in the Vedic literature to indicate Dharma.
Vṛṣa also means virtue. So the bull is equated with Dharma. So it should be taken that Shiva not only rides on a bull. But he rides on Virtue.
Gods are not carried by same Vahana all the time.
It is said that Goddess Lakshmi's mount is an Owl. But it is not the same all the time. She takes different mounts at different times like a swan, lotus etc.,.
Also Indra also owns a horse named Uchhaishrava. He is not always carried by the elephant Airavata.
Vishnu usually rides on a mighty eagle named Garuda. But he is carried by a mighty serpent named Adi Shesha. So, in a way he is also a carrier(Vahana).
Ganesha has a mouse as his mount. Usually a rat or a mouse is recognised as a stealer and one with unstable mentality. Taking control over the mouse shows that he is not the one with unstable behavior. It also teaches that we should control our unstable behavior by worshipping Ganesha.
Why is only Kubera associated with a man as his vahana?
Here Kubera is associated with human carriage. It could mean that he has control over the Human. Because humans are very much attracted to money.